Method of preventing epitaxy creeping under the spacer

ABSTRACT

After forming a gate spacer on each sidewall of a sacrificial gate structure, portions of each dielectric fin cap portion underneath the gate spacer is intentionally etched and undercut regions that are formed are filled and pinched off with a dielectric material of a conformal dielectric liner. Portions of the conformal dielectric liner in the undercut regions are not subject to the undercut during an epitaxial pre-clean process performed prior to forming an epitaxial source region and an epitaxial drain region on opposite sides of the sacrificial gate structure and remain in the undercut regions after forming the epitaxial source region and the epitaxial drain region.

BACKGROUND

The present disclosure relates to semiconductor device fabrication, andmore particularly to a method to prevent undesired undercutting ofdielectric fin caps during fin field effect transistor (FinFET)fabrication in a replacement gate scheme.

Fin field effect transistors (FinFETs) are one of the most promisingcandidates for scaling beyond 32 nm node. A FinFET typically includes asource region and a drain region interconnected by fins which serve as achannel region of the device and a gate that surrounds at least aportion of each of the fins between the source region and the drainregion. Epitaxial deposition is typically used to form the source regionand the drain region. Current epitaxial deposition processes typicallyrequire a pre-clean process to remove surface defects or contaminationsresiding on the starting surface of a semiconductor substrate targetedfor deposition. The conventional pre-clean process generally uses eitheran aqueous solution of hydrogen fluoride (HF), or a gas phase HF toremove the surface defects or contaminations.

In fabricating FinFETs, it is desirable to have a relatively thick oxide(or other dielectric material) cap on top of each fin to prevent damagesto the fins during device processing especially during gate spacerformation which typically involves a reactive ion etch (RIE). However,when the epitaxial deposition is applied for fabricating a source regionand a drain region, portions of the thick oxide cap underneath the gatespacers are susceptible to undercut during the extensive epitaxypre-clean process. The undercut leads to epitaxy creeping under thespacer and excessive dopant diffusion at the top of the fins. As such, amethod to prevent undercutting of the oxide capping layer during theepitaxy pre-clean process is needed.

SUMMARY

The present disclosure provides a method to prevent undesiredundercutting of dielectric fin caps during fin field effect transistor(FinFET) fabrication in a replacement gate scheme. After forming a gatespacer on each sidewall of a sacrificial gate structure, portions ofeach dielectric fin cap portion underneath the gate spacers areintentionally etched and undercut regions that are formed are filled andpinched off with a dielectric material of a conformal dielectric liner.Portions of the conformal dielectric liner in the undercut regions arenot subject to the undercut during an epitaxial pre-clean processperformed prior to forming an epitaxial source region and an epitaxialdrain region on opposite sides of the sacrificial gate structure andremain in the undercut regions after forming the epitaxial source regionand the epitaxial drain region.

In one aspect of the present disclosure, a method of forming asemiconductor structure is provided. In one embodiment, the methodincludes etching a semiconductor substrate to form a plurality of finstructures. Each of the plurality of the fin structures has a dielectricfin cap atop the fin structure. A sacrificial gate structure is thenformed over the dielectric fin caps. A gate spacer is then formed oneach sidewall of the sacrificial gate structure. Next, a dielectric fincap portion is formed by removing portions of each of the dielectric fincaps that are not covered by the sacrificial gate structure or the gatespacer. Each of the dielectric fin cap portions is then laterally etchedto provide an undercut region underneath each gate spacer. A dielectricliner portion is formed within each of the undercut regions. Next, anepitaxial source region and an epitaxial drain region are formed onportions of the fin structures that are not covered by the sacrificialgate structure, the gate spacers and the dielectric liner portions.After removing the sacrificial gate structure to expose remainingportions of the dielectric fin cap portions, the remaining portions ofthe dielectric fin cap portions are removed to form a gate cavity. Thedielectric liner portions remain in the undercut regions during theforming of the epitaxial source region and the epitaxial drain region.

In another aspect of the present disclosure, a semiconductor structureis provided. The semiconductor structure includes a gate structuredisposed on a channel portion of a fin structure, a gate spacer disposedon each sidewall of the gate structure, and a dielectric liner portiondisposed underneath the gate spacer and in contact with the finstructure. The dielectric liner portion has a width less than a width ofthe gate spacer and an outer sidewall aligned with an outer sidewall ofthe gate spacer. The semiconductor structure further includes anepitaxial source region and an epitaxial drain region adjacent the gatespacers and the dielectric liner portions. The epitaxial source regionand the epitaxial drain region contact portions of the fin structureexposed by the gate structure, the gate spacers and the dielectric linerportions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top-down view of an exemplary semiconductor structureincluding a semiconductor substrate and a dielectric capping layerformed thereon that can be employed in accordance with an embodiment ofthe present disclosure.

FIG. 1B is a cross sectional view of the exemplary semiconductorstructure of FIG. 1A along line B-B′.

FIG. 2A is a top-down view of the exemplary semiconductor structure ofFIG. 1A after forming a plurality of fin structures with a dielectricfin cap atop of each fin structure.

FIG. 2B is a cross sectional view of the exemplary semiconductorstructure of FIG. 2A along line B-B′.

FIG. 3A is a top-down view of the exemplary semiconductor structure ofFIG. 2A after forming a sacrificial gate stack that is orientedperpendicular to each fin structure.

FIG. 3B is a cross sectional view of the exemplary semiconductorstructure of FIG. 3A along line B-B′.

FIG. 4A is a top-down view of the exemplary semiconductor structure ofFIG. 3A after forming a gate spacer on each sidewall of the sacrificialgate stack.

FIG. 4B is a cross sectional view of the exemplary semiconductorstructure of FIG. 4A along line B-B′.

FIG. 5A is a top-down view of the exemplary semiconductor structure ofFIG. 4A after undercutting and removing portions of the dielectric fincap portions to provide undercut regions.

FIG. 5B is a cross sectional view of the exemplary semiconductorstructure of FIG. 5A along line B-B′.

FIG. 6A is a top-down view of the exemplary semiconductor structure ofFIG. 5A after forming a conformal dielectric liner over the sacrificialgate stack, the fin structures and a buried insulator layer of thesemiconductor substrate.

FIG. 6B is a cross sectional view of the exemplary semiconductorstructure of FIG. 6A along line B-B′.

FIG. 7A is a top-down view of the exemplary semiconductor structure ofFIG. 6A after forming dielectric liner portions.

FIG. 7B is a cross sectional view of the exemplary semiconductorstructure of FIG. 7A along line B-B′.

FIG. 8A is a top-down view of the exemplary semiconductor structure ofFIG. 7A after forming a source region and a drain region adjacent thegate spacers, the dielectric liner portions and the fin structures.

FIG. 8B is a cross sectional view of the exemplary semiconductorstructure of FIG. 8A along line B-B′.

FIG. 9A is a top-down view of the exemplary semiconductor structure ofFIG. 8A after forming an interlevel dielectric layer over thesacrificial gate stack, the gate spacers, the source region, the drainregion and the buried insulator layer.

FIG. 9B is a cross sectional view of the exemplary semiconductorstructure of FIG. 8B along line B-B′.

FIG. 10A is a top-down view of the exemplary semiconductor structure ofFIG. 9A after removing the sacrificial gate stack and the recesseddielectric fin cap portions to form a gate cavity.

FIG. 10B is a cross sectional view of the exemplary semiconductorstructure of FIG. 9B along line B-B′.

FIG. 11A is a top-down view of the exemplary semiconductor structure ofFIG. 10A after forming a gate dielectric and a gate electrode within thegate cavity.

FIG. 11B is a cross sectional view of the exemplary semiconductorstructure of FIG. 10B along line B-B′.

DETAILED DESCRIPTION

The present disclosure will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent disclosure. It is noted that the drawings of the presentdisclosure are provided for illustrative purposes only and, as such, thedrawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present disclosure. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present disclosure may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present disclosure.

Referring to FIGS. 1A and 1B, there is illustrated an exemplarysemiconductor structure that can be employed in one embodiment of thepresent disclosure. Specifically, the semiconductor structure includes asemiconductor substrate 8 having a dielectric capping layer 16L formedthereon. In one embodiment of the present disclosure and as shown inFIG. 1, the semiconductor substrate 8 is a semiconductor-on-insulator(SOI) substrate including a handle substrate 10, a buried insulatorlayer 12 and a top semiconductor layer 14.

In some embodiments of the present disclosure, the handle substrate 10can include a semiconductor material, such as, for example, Si, Ge,SiGe, SiC, SiGeC, and III/V compound semiconductors. In otherembodiments, the handle substrate 10 may be a dielectric material or aconductive material. In further embodiments, the handle substrate 10 canbe omitted. When present, the handle substrate 10 provides mechanicalsupport to the buried insulator layer 12 and the top semiconductor layer14. The thickness of the handle substrate 10 can be from 30 μm to about2 mm, although less and greater thicknesses can also be employed.

The buried insulator layer 12 includes a dielectric material such assilicon oxide, silicon nitride, silicon oxynitride, or a combinationthereof. The thickness of the buried insulator layer 12 can be from 50nm to 200 nm, with a thickness from 100 nm to 150 nm being more typical.

The top semiconductor layer 14 can include a semiconductor material suchas, for example, Si, Ge, SiGe, SiC, SiGeC, and III/V compoundsemiconductors such as, for example, InAs, GaAs, and InP. Thesemiconductor materials of the top semiconductor layer 14 and the handlesubstrate 10 may be the same or different. Typically, each of the handlesubstrate 10 and the top semiconductor layer 14 comprises a singlecrystalline semiconductor material, such as, for example, singlecrystalline silicon. The top semiconductor layer 14 may or may not bedoped with p-type dopants and/or n-type dopants. The thickness of thetop semiconductor layer 14 can be from 10 nm to 200 nm, with a thicknessfrom 30 nm to 70 nm being more typical.

In another embodiment of the present disclosure, the semiconductorsubstrate 8 is a bulk substrate (no shown). When a bulk semiconductorsubstrate is employed as semiconductor substrate 8, the bulksemiconductor substrate can be comprised of any semiconductor materialincluding, but not limited to, Si, Ge, SiGe, SiC, SiGeC, and III/Vcompound semiconductors such as, for example, InAs, GaAs, and InP.Multilayers of these semiconductor materials can also be used as thesemiconductor material of the bulk semiconductor. In some embodiments,the semiconductor substrate 8 comprises a single crystallinesemiconductor material, such as, for example, single crystallinesilicon. In other embodiments, the semiconductor substrate 8 maycomprise a polycrystalline or amorphous semiconductor material.

The dielectric capping layer 16L can be formed on a top surface of thetop semiconductor layer 14. During the subsequent formation of the finstructures, a portion of the dielectric capping layer 16L provides adielectric fin cap atop each fin structure. In one embodiment, thedielectric capping layer 16L includes an oxide. Examples of oxides thatcan be used as the dielectric capping layer 16L include, but are notlimited to silicon oxide, zirconium oxide, tantalum oxide, hafnium oxideand aluminum oxide. The dielectric capping layer 16L can be formed byconventional deposition processes, such as, for example chemical vapordeposition (CVD) or plasma-enhanced CVD (PECVD). Alternatively, athermal growing process such as thermal oxidation may be used in formingthe dielectric capping layer 16L. The thickness of the dielectriccapping layer 16L can be from 5 nm to 20 nm, although lesser and greaterthicknesses can also be employed.

Referring to FIGS. 2A and 2B, there are illustrated the exemplarysemiconductor structure of FIGS. 1A and 1B after forming a plurality offin structures 18. In the illustrated embodiment of the presentdisclosure, each of the fin structures 18 has a bottommost surface thatis located on a top surface of the buried insulator layer 12 of the SOIsubstrate.

The fin structures 18 can be formed by lithography and etching.Lithography can include forming a photoresist (not shown) on a topsurface of the dielectric capping layer 16L, exposing the photoresist toa desired pattern of radiation, and then developing the exposedphotoresist with a conventional resist developer to provide a patternedphotoresist atop the dielectric capping layer 16L. A two-stageanisotropic etch may be conducted to transfer the pattern from thepatterned photoresist into the dielectric capping layer 16L and the topsemiconductor layer 14. In one embodiment, the etch used for patterntransfer may include a dry etch process such as, for example, RIE,plasma etching, ion beam etching or laser ablation. During the firststage of the anisotropic etch, the exposed portions of the dielectriccapping layer 16L are removed selective to the top semiconductor layer14, exposing portions of the top semiconductor layer 14. Remainingportions of the dielectric capping layer 16L after the lithographicpatterning constitute a plurality of dielectric fin caps 16 and functionas an etch mask that protects the underlying portions of the topsemiconductor layer 14 from which the fin structures 18 are subsequentlyformed. During the second stage of the anisotropic etch, the exposedportions of the top semiconductor layer 14 are removed selective to theburied insulator layer 12 utilizing the underlying buried insulatorlayer 12 as an etch stop. Remaining portions of the top semiconductorlayer 14 after the lithographic patterning constitute a plurality of finstructures 18. In one embodiment, each of the fin structure 18 iscomposed of silicon, and each dielectric fin cap 16 that is atop each ofthe fin structures 18 is composed of silicon oxide. After transferringthe pattern into the dielectric capping layer 16L and the topsemiconductor layer 14, the patterned photoresist can be removedutilizing a conventional resist stripping process such as, for example,ashing. Other methods known in the art such as sidewall image transfer(SIT) or directional self-assembly (DSA) can be used to pattern the finstructures.

Each of the fin structures 18 that are formed may have a height rangingfrom 5 nm to 200 nm, with a height ranging from 10 nm to 100 nm beingmore typical. Each of the fin structures 18 may have a width rangingfrom 4 nm to 50 nm, with a width ranging from 5 nm to 20 nm being moretypical. Adjacent fin structures 18 may be separated by a pitch rangingfrom 20 nm to 100 nm. In one embodiment, the adjacent fin structures areseparated by a pitch ranging from 30 nm to 50 nm.

Referring now to FIGS. 3A and 3B, there are illustrated the exemplarysemiconductor structure of FIGS. 2A and 2B after forming a sacrificialgate stack that is oriented perpendicular to and straddles each finstructure 18. The sacrificial gate stack includes, from bottom to top, asacrificial gate structure 22 and a sacrificial gate cap 24. The term“sacrificial gate structure” as used herein refers to a placeholderstructure for a functional gate structure to be subsequently formed. Theterm “functional gate structure” as used herein refers to a permanentgate structure used to control output current (i.e., flow of carriers inthe channel) of a semiconducting device through electrical fields.Although only one sacrificial gate stack is described and illustrated,the present disclosure is not limited to such a number of sacrificialgate stacks. Instead, a plurality of sacrificial gate stacks can beformed upon portions of the semiconductor substrate 8.

The sacrificial gate structure 22 and the sacrificial gate cap 24 can beformed by depositing a sacrificial gate material layer (not shown) and asacrificial gate capping layer (not shown), and subsequentlylithographically patterning the sacrificial gate material layer and thesacrificial gate capping layer. A remaining portion of the sacrificialgate material layer after the lithographic patterning constitutes thesacrificial gate structure 22, and a remaining portion of thesacrificial gate capping layer after the lithographic patterningconstitutes the sacrificial gate cap 24.

The sacrificial gate material layer includes a material that can beremoved selective to the material of the dielectric fin caps 16. Thesacrificial gate material layer can include a semiconductor material, adielectric material that is different from the dielectric material ofthe dielectric fin caps 16, or a metallic material. Exemplarysemiconductor materials that can be employed for the sacrificial gatematerial layer include silicon, germanium, a silicon germanium alloy, asilicon carbon alloy, a compound semiconductor material, or acombination thereof. Exemplary metallic materials that can be employedfor the sacrificial gate layer include W, Co, TiN, TaN, and TiC. Thesacrificial gate material layer can be deposited, for example, by CVD orPECVD. The thickness of the sacrificial gate material layer, as measuredabove a planar surface, can be from 50 nm to 300 nm, although lesser andgreater thicknesses can also be employed.

The sacrificial gate capping layer may be composed of an oxide, nitrideor oxynitride. In one embodiment, the sacrificial gate capping layerincludes silicon nitride. The sacrificial gate capping layer can beformed using CVD, physical vapor deposition (PVD), thermal growthmethods, or a combination thereof. The thickness of the sacrificial gatecap layer can be from 20 nm to 300 nm, although lesser and greaterthicknesses can also be employed.

Referring now to FIGS. 4A and 4B, there are illustrated the exemplarysemiconductor structure of FIGS. 3A and 3B after forming a gate spaceron each sidewall of the sacrificial gate stack (22, 24). A conformalspacer material layer (not shown) can be deposited on the sacrificialgate stack of the sacrificial gate structure 22 and the sacrificial gatecap 24 and over the dielectric fin caps 16, for example, by CVD or ALD.The conformal spacer material layer includes a dielectric material thatis different from the dielectric material of the dielectric fin caps 16.In one embodiment and when the dielectric fin caps 16 include adielectric oxide, the conformal spacer material is a dielectric nitride.Examples of the dielectric nitride include, but are not limited to,silicon nitride, and silicon oxynitride. In other embodiments, a low-kdielectric, such as, for example, SiBN and SiCN can be employed to formthe conformal spacer material layer. The thickness of the conformalspacer material layer can be from 3 nm to 20 nm, although lesser andgreater thicknesses can also be employed.

Horizontal portions of the conformal spacer material layer aresubsequently removed by an anisotropic etch, such as, for example, a RIEprocess. In one embodiment of the present disclosure, the RIE process iscontinued so that vertical portions of the conformal spacer materiallayer that are formed on the sidewalls of the fin structures 18 areremoved. The dielectric fin caps 16 protect the underlying finstructures 18 during the RIE process. The remaining vertical portions ofthe dielectric material layer constitute the gate spacers 26.

During the above mentioned anisotropic etch, portions of the dielectricfin caps 16 that are not covered by the sacrificial gate stack (22, 24)or the gate spacers 26 are typically also removed. If the anisotropicetch used in forming the gate spacers 26 does not remove the underlyingdielectric fin caps 16, a separating etching process can be used toselectively remove the portions of the dielectric fin caps 16 that arenot covered by the sacrificial gate stack (22, 24) or the gate spacers26. The separate etching process may employ the same etch chemistry asthe etch that forms the gate spacers 26, or may employ a different etchchemistry. The separate etch can be selective to the semiconductormaterial of the fin structures 18. The separate etch can be ananisotropic etch or an isotropic etch. Remaining portions of thedielectric fin caps 16 that are covered by the sacrificial gate stack(22, 24) and the gate spacers 26 are herein referred to as dielectricfin cap portions 16A.

Referring to FIGS. 5A and 5B, there are illustrated the exemplarysemiconductor structure of FIGS. 4A and 4B after undercutting andremoving portions of the dielectric fin cap portions 16A to provideundercut regions 30. As is shown in FIG. 5B, the sidewalls of thedielectric fin cap portions 16A are laterally etched by a lateraldistance. The lateral distance is typically no greater than the width ofthe gate spacers 26. The openings thus formed, which are defined by gapsbetween the gate spacers 26 and the fin structures 18, are hereinreferred to as undercut regions 30. In one embodiment, the lateraldistance can be from 2 nm to 10 nm. The remaining portions of thedielectric fin cap portions 16A after the lateral undercut are hereinreferred to as recessed dielectric fin cap portions 16B.

The lateral etch can be provided by an isotropic etch, such as a wetetch having an etch selectivity for removing the dielectric fin capportions 16A selective to the fin structures 18 and the gate spacers 26.This process laterally etches portions of the dielectric fin capportions 16A underneath the spacers 26. In one embodiment, when thedielectric fin cap portions 16A is composed of silicon oxide and eachgate spacer 26 is composed of silicon nitride, an isotropic etch, suchas a wet etch using dilute hydrofluoric acid (DHF), may be used toselectively undercut the dielectric fin cap portions 16A.

Referring to FIGS. 6A and 6B, there are illustrated the exemplarysemiconductor structure of FIGS. 5A and 5B after forming a conformaldielectric liner 32L over the sacrificial gate stack (22, 24), the finstructures 18 and the buried insulator layer 12. The conformaldielectric liner 32L can be formed by using conventional depositiontechniques, such as CVD or ALD and may be comprised of the same materialas the gate spacers 26. The conformal dielectric liner 32L can compriseany suitable dielectric material such as, for example, silicon nitride,silicon oxynitride, or a low-k dielectric such as SiBN or SiCN. In oneembodiment, the dielectric liner 32L may be comprised of siliconnitride. As the thickness of dielectric liner 32L increases during thedeposition process, the undercut regions 30 are filled and pinched offwith the dielectric material of the conformal dielectric liner 32L.

Referring to FIGS. 7A and 7B, there are illustrated the exemplarysemiconductor structure of FIGS. 6A and 6B after forming dielectricliner portions 32. The dielectric liner portions 32 can be formed byremoving portions of the conformal dielectric liner 32L that are notcovered by the gate spacers 26 utilizing an isotropic etch, such as, forexample, a wet etch process, and leaving portions of the conformaldielectric liner 32L in the undercut regions 30 intact. Remainingportions of the conformal dielectric liner 32L are herein referred to asdielectric liner portions 32. In one embodiment, the isotropic etch mayremove the material of the conformal dielectric liner 32L selective tomaterials of the sacrificial gate cap 24, the gate spacers 26, the finstructures 18 and the buried insulator layer 12. In another embodiment,where the conformal dielectric liner 32L, the sacrificial gate cap 24and the gate spacers 26 are composed of the same material, such as, forexample, silicon nitride, the etch process can be timed so as to removethe conformal dielectric liner 32L with the exception of the pinched offregion formed under the gate spacers 26. Each of the dielectric linerportions 32 that are formed has an outer sidewall that is aligned withan outer sidewall of each gate spacer 26. In one embodiment, each of thedielectric liner portions 32 has a width from 3 nm to 5 nm.

Referring to FIGS. 8A and 8B, there are illustrated the exemplarysemiconductor structure of FIGS. 7A and 7B after forming a source regionand a drain region (collectectively referred to herein after as sourceand drain regions 42) adjacent the gate spacers 26, the dielectric linerportions 32 and the fin structures 18. The source and drain regions 42can be formed by epitaxially depositing a semiconductor material overthe exposed surfaces of fin structures 18, but not on dielectricsurfaces such as the surfaces of the sacrificial gate cap 24, the gatespacer 26 and the buried insulator layer 12.

Prior to the epitaxial deposition of the semiconductor material, aseries of wet cleans, dry cleans or other physical cleaning techniquesmay be performed to remove contaminants, such as, for example, resistresidues and any remaining oxides from the exposed surfaces of the finstructures 18. The dielectric liner portions 32 remain in the undercutregions 30 after the pre-epitaxial clean processes.

The epitaxial grown of the semiconductor material can be effected byplacing the exemplary semiconductor structure of FIGS. 7A and 7B into areaction chamber, and simultaneously, or alternately, flowing at leastone reactant gas (such as SiH₄, Si₂H₆, SiH₂Cl₂, GeH₄, C₂H₂, C₂H₄) and anetchant gas (such as HCl) into the reaction chamber. Optionally, acarrier gas such as H₂, N₂, H₂, and/or Ar can be flowed into thereaction chamber. The temperature for epitaxial deposition typicallyranges from 550° C. to 1300° C. The apparatus for performing theepitaxial growth may include a CVD apparatus, such as atmosphericpressure CVD (APCVD), low pressure CVD (LPCVD) and PECVD.

The semiconductor material that can be epitaxially deposited includesany semiconductor material such as, for example, Si, SiGe and Si:C. Inone embodiment, when the semiconductor structure of the presentdisclosure is a pFET, the source and drain regions 42 are comprised ofSiGe. In another embodiment, when the semiconductor structure of thepresent disclosure is an nFET, the source and drain regions 42 arecomprised of Si or Si:C.

The semiconductor material of the source and drain regions 42 can bedeposited as an intrinsic semiconductor material, or can be depositedwith in-situ doping. If the semiconductor material is deposited as anintrinsic semiconductor material, the source and drain regions 42 can besubsequently doped (ex-situ) utilizing ion implantation, gas phasedoping or dopant out diffusion from a sacrificial dopant sourcematerial. For a pFET, the source and drain regions 42 are doped with ap-type dopant and for an nFET, the source and drain regions 42 are dopedwith an n-type dopant. Examples of p-type dopants include, but are notlimited to, B, Al, Ga or In. Examples of n-type dopants include, but arenot limited to, P, As or Sb. The p-type dopants or n-type dopants can beactivated subsequently using a rapid thermal process.

Referring to FIGS. 9A and 9B, there are illustrated the exemplarysemiconductor structure of FIGS. 8A and 8B after forming an interleveldielectric layer 50 over the sacrificial gate cap 24, the gate spacers26, the source and drain regions 42 and the exposed portions of theburied insulator layer 12. The interlevel dielectric layer 50 includesat least one dielectric material, which can be silicon oxide, siliconnitride, silicon oxynitride, an organosilicate glass (OSG), or acombination thereof. The thickness of the interlevel dielectric layer 50can be selected so that an entirety of the top surface of the interleveldielectric layer 50 is formed above the top surface of the sacrificialgate cap 24.

The interlevel dielectric layer 50 can be subsequently planarized, forexample, by chemical mechanical planarization (CMP) and/or a recessetch. In one embodiment, the sacrificial gate cap 24 can be employed asan etch stop.

Referring to FIGS. 10A and 10B, there are illustrated the exemplarysemiconductor structure of FIGS. 9A and 9B after removing thesacrificial gate cap 24, the sacrificial gate structure 22 and therecessed dielectric fin cap portions 16B to provide a gate cavity 52.

Additional portions of the interlevel dielectric layer 50 and thesacrificial gate cap 24 can be removed by an additional planarizationprocess, which can be performed by additional CMP and/or an additionalrecess etch. In one embodiment, the sacrificial gate structure 22 can beemployed as an etch stop and after removing the sacrificial gate cap 24,a top surface of the sacrificial gate structure 22 is physically exposed(not shown). The top surface of the interlevel dielectric layer 50 canbe a planar surface.

The sacrificial gate structure 22 is then removed selective to therecessed dielectric fin cap portions 16B. The sacrificial gate structure22 can be removed using a wet chemical etch or a dry etch. In oneembodiment and when the sacrificial gate structure 22 is composed ofpolysilicon, the sacrificial gate structure 22 can be removed using asilicon-specific RIE process.

Next, the recessed dielectric fin cap portions 16B are removed selectiveto the fin structures 18. The recessed dielectric fin cap portions 16Bcan be removed using, for example, an isotropic RIE. A gate cavity 52 isformed within a volume from which the sacrificial gate structure 22 andthe recessed dielectric fin cap potions 16B are removed and is laterallyconfined by inner sidewalls of the gate spacers 26 and the dielectricliner portions 32. In one embodiment, the gate cavity 52 has an invertedT-shape.

Referring to FIGS. 11A and 11B, there are illustrated the exemplarysemiconductor structure of FIGS. 10A and 10B after forming a gatedielectric 54 and a gate electrode 56 within the gate cavity 52. Thecombination of the gate dielectric 54 and the gate electrode 56 definesa functional gate structure. In one embodiment, the functional gatestructure has an inverted T-shape.

The gate dielectric 54 can be formed directly on each lengthwisesidewall of the fin structures 18. The gate dielectric 54 can be formedas a conformal layer, for example, by deposition of a dielectricmaterial by CVD or ALD, or by conversion of surface portions of the finstructures 18 into a dielectric material such as silicon oxide, siliconnitride, or silicon oxynitride. In one embodiment, the gate dielectric54 is composed of a high-k material having a dielectric constant greaterthan silicon oxide. Exemplary high-k materials include, but are notlimited to, HfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃,HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y),SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), SiON, SiN_(x), a silicatethereof, and an alloy thereof. Each value of x is independently from 0.5to 3 and each value of y is independently from 0 to 2.

The gate electrode 56 can be formed by filling remaining portions of thegate cavity 52 with at least one conductive material such as at leastone metallic material and/or at least one doped semiconductor material.Examples of the conductive metal include, but are not limited to, Al, W,Cu, Pt, Ag, Au, Ru, Ir, Rh and Re, alloys of a conductive metal, e.g.,Al—Cu, metal nitrides or carbides such as AN, TiN, TaN, TiC and TaC,silicides of a conductive metal, e.g., W silicide, and Pt silicide, andcombinations thereof. The gate electrode 56 can be formed by depositingthe conductive material utilizing a conventional deposition process suchas, for example, ALD, CVD, metalorganic chemical vapor deposition(MOCVD), molecular beam epitaxy (MBE), PVD, sputtering, plating,evaporation, ion beam deposition, electron beam deposition, laserassisted deposition, and chemical solution deposition. The portion ofthe at least one conductive material above the top surface of theinterlevel dielectric layer 50 can be removed, for example, by chemicalmechanical planarization. The portion of the gate dielectric 52, ifformed above the top surface of the interlevel dielectric layer 50, may,or may not, be subsequently removed.

While the present disclosure has been particularly shown and describedwith respect to various embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A method of forming a semiconductor structurecomprising: etching a semiconductor substrate to form a plurality of finstructures, each of the plurality of the fin structures having adielectric fin cap atop the fin structure; forming a sacrificial gatestructure over the dielectric fin caps; forming a gate spacer on eachsidewall of the sacrificial gate structure; forming a dielectric fin capportion by removing portions of each of the dielectric fin caps that arenot covered by the sacrificial gate structure or the gate spacer;laterally etching each of the dielectric fin cap portions to provide anundercut region underneath each gate spacer; forming a dielectric linerportion within each of the undercut regions; forming an epitaxial sourceregion and an epitaxial drain region on portions of the fin structuresthat are not covered by the sacrificial gate structure, the gate spacersand the dielectric liner portions; removing the sacrificial gatestructure to expose remaining portions of the dielectric fin capportions; and removing the remaining portions of the dielectric fin capportions to form a gate cavity, wherein the dielectric liner portionsremain in the undercut regions during said forming the epitaxial sourceregion and the epitaxial drain region.
 2. The method of claim 1, whereinthe undercut region has a width no greater than a width of the gatespacers
 3. The method of claim 1, wherein the dielectric liner portionhas an outer sidewall aligned with an outer sidewall of the gate spacer.4. The method of claim 1, wherein said etching the semiconductorsubstrate to form the plurality of fin structures comprises: forming adielectric capping layer on a uppermost surface of the semiconductorsubstrate; forming a mask on the dielectric capping layer; etchingportions of the dielectric capping layer not covered by the mask to forma plurality of dielectric fin caps; and anisotropically etching thesemiconductor substrate using the plurality of the dielectric fin capsas an etch mask to form the plurality of the fin structures.
 5. Themethod of claim 1, wherein the dielectric fin cap comprises siliconoxide.
 6. The method of claim 1, wherein the dielectric liner portioncomprises a same dielectric material as the gate spacer.
 7. The methodof claim 6, wherein the gate spacer and the dielectric liner portioncomprise silicon nitride, silicon oxynitride, or a low-k dielectricselected from the group consisting of SiBN and SiCN.
 8. The method ofclaim 1, wherein said forming the sacrificial gate structure comprises:forming a sacrificial gate material layer over the dielectric fin capsand the semiconductor structure; and patterning the sacrificial gatematerial layer.
 9. The method of claim 8, further comprising forming asacrificial gate cap on the sacrificial gate structure.
 10. The methodof claim 1, wherein said forming the gate spacer comprises: depositing aconformal spacer material layer on the sacrificial gate structure andthe dielectric fin caps; and removing horizontal portions of theconformal spacer material layer and vertical portions of the spacermaterial layer on sidewalls of each fin structure.
 11. The method ofclaim 1, wherein said forming the dielectric liner portion in eachundercut region comprises: forming a conformal dielectric liner over thegate structure, the gate spacers, portions of the fin structures thatare not covered by the gate structure or the gate spacers and exposedportions of the semiconductor substrate, wherein during forming saidconformal dielectric liner, the undercut regions are filled and pinchedoff with a dielectric material of the conformal dielectric liner; andremoving portions of the conformal dielectric liner that are not coveredby the gate spacers.
 12. The method of claim 1, further comprisingperforming an epitaxial pre-clean process on the portions of the finstructures that are not covered by the sacrificial gate structure, thegate spacers and the dielectric liner portions prior to said forming theepitaxial source region and the epitaxial drain region.
 13. The methodof claim 12, wherein the epitaxial pre-clean process comprises a wetclean process, a dry clean process, or a combination thereof.
 14. Themethod of claim 1, wherein said removing the sacrificial gate structurecomprises: forming an interlevel dielectric layer over exposed portionsof the plurality of the fin structures and the semiconductor substrate,wherein the interlevel dielectric layer has an upper surface coplanarwith an upper surface of the sacrificial gate structure; and removingthe sacrificial gate structure with an etch that is selective to theplurality of the fin structures, the epitaxial source region, theepitaxial drain region and the interlevel dielectric layer.
 15. Themethod of claim 1, further comprising forming a gate structure in thegate cavity, wherein said forming the gate structure comprises: forminga gate dielectric directly on sidewalls and a bottom surface of the gatecavity; and forming a gate electrode on the gate dielectric to fill aremaining volume of the gate cavity.
 16. A semiconductor structurecomprising: a gate structure disposed on a channel portion of a finstructure; a gate spacer disposed on each sidewall of the gatestructure; a dielectric liner portion disposed underneath the gatespacer and in contact with the fin structure, wherein the dielectricliner portion has a width less than a width of the gate spacer and anouter sidewall aligned with an outer sidewall of the gate spacer; and anepitaxial source region and an epitaxial drain region adjacent the gatespacers and the dielectric liner portions, the epitaxial source regionand the epitaxial drain region contacting portions of the fin structureexposed by the gate structure, the gate spacers and the dielectric linerportions.
 17. The semiconductor structure of claim 16, wherein the gatestructure has an inverted T-shape in which a portion of the gatestructure contacting the dielectric liner portion has a width greaterthan a width of a remaining portion of the gate structure.
 18. Thesemiconductor structure of claim 16, wherein the gate spacer and thedielectric liner portion comprise a same dielectric material.
 19. Thesemiconductor structure of claim 16, wherein the epitaxial sourceregions and the epitaxial drain region comprise a doped semiconductormaterial.
 20. The semiconductor structure of claim 16, wherein the gatestructure comprises a gate dielectric and a gate electrode.